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Effect of cyclic solution treatment on microstructure and mechanical properties of friction stir welded 7075 Al alloy
Author’s Accepted Manuscript Effect of cyclic solution treatment on microstructure and mechanical properties of friction stir welded 7075 Al alloy S.M. Bayazid, H. Farhangi, H. Asgharzadeh, L. Radan, A. Ghahramani, A. Mirhaji www.elsevier.com
PII: DOI: Reference:
S0921-5093(15)30473-1 http://dx.doi.org/10.1016/j.msea.2015.10.010 MSA32861
To appear in: Materials Science & Engineering A Received date: 10 July 2015 Revised date: 2 October 2015 Accepted date: 3 October 2015 Cite this article as: S.M. Bayazid, H. Farhangi, H. Asgharzadeh, L. Radan, A. Ghahramani and A. Mirhaji, Effect of cyclic solution treatment on microstructure and mechanical properties of friction stir welded 7075 Al alloy, Materials Science & Engineering A, http://dx.doi.org/10.1016/j.msea.2015.10.010 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Effect of cyclic solution treatment on microstructure and mechanical properties of friction stir welded 7075 Al alloy
S.M. Bayazid1*, H. Farhangi2, H. Asgharzadeh3, L. Radan4, A. Ghahramani5, A. Mirhaji6
1,2,5
School of Metallurgy and Materials, College of Engineering, University of Tehran,
Tehran, P.O. 11155-4563, Iran 3,6
Department of Materials Engineering, University of Tabriz, P.O. Box 51666-16471,
Tabriz, Iran 4
Department of Materials Science and Engineering, School of Engineering, Shiraz
University, P.O. Box 71348-51154, Shiraz, Iran
* Corresponding author. Email: [email protected] Cell phone number: +98 9141485543
1
Abstract 7075-T6 aluminum alloy plates were prepared by friction stir welding (FSW) followed by age hardening. A novel solutionizing method, namely cyclic solution treatment (CST), comprising of a repeated heating between 400 and 480 ˚C for 0.25 h was employed. The microstructure of the joints was studied by optical microscopy (OM), scanning electron microscopy (SEM) and transmission electron microscopy (TEM) techniques. The effect of CST on mechanical properties was assessed by means of tensile test and microhardness measurement. A significant grain size refinement is taken place by FSW whilst the grain size is not considerably changed after CST. The results show that precipitate particles of the welding area before and after heat treatment are MgZn2 and MgAlCu/Al7Cu2Fe, respectively. CST improves tensile strength and elongation while homogenizes the hardness distribution of the FSWed joint. A noteworthy enhancement in the hardness (~ 45%) and tensile strength (~ 33%) of the FSWed sample is achieved after CST and aging at 130 ˚C for 24 h. The tensile fracture surface of the Al alloy joint demonstrates fine dimples after CST while lessdeveloped dimples are detected after aging.
Keywords: 7075 Al alloy; Friction stir welding; Cyclic solution treatment; Aging; Microstructure; Mechanical properties
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1. Introduction 7xxx series aluminum alloys have a wide variety of applications in different industries such as aerospace and automotive due to their high strength-to-weight ratio. However, these alloys are difficult to join with fusion welding methods which produce solidification cracks and high volume of porosities [1-4]. Accordingly, friction stir welding (FSW) as a solid state joining method is being noticed recently by many industries and researchers due to its capability to reduce defects compared with conventional techniques [5]. Although there is a large body of research in the literature on FSW of 7075 Al alloy [3, 5, 6], however, many problems still remain unsolved related to poor mechanical properties of weldment’s heat affected zone (HAZ) [7-9]. Recently, some efforts have been devoted to improving the mechanical properties of the FSWed joints like changing the welding and tooling parameters (e.g. rotational and longitudinal speed, size of the pin and geometry of the tool) [10, 11] and applying shot-peening or laser-peening prior to welding [12]. Among the various methods, post-weld heat treatment (PWHT) is a versatile and inexpensive technique for the recovery of degraded mechanical properties of FSWed parts [13-15]. Sato and Kakawa [15] have shown that the yield and tensile strength of FSWed 6063-T5 Al alloy increased after solution treatment at 530 ˚C for 1 h followed by water quenching and aging at 175 ˚C for 12 h. Feng et al. [13] have reported that PWHT of FSWed 2219-O alloy has a considerable effect on the fracture locations of the joints. Meanwhile, the tensile strength of the weldment can be improved by increasing the solutionizing temperature. Priya et al. [14] have stated that aging of FSWed 2219 and 6061-T6 alloys at 165 ˚C for 18 h without solution treatment results in a considerable improvement in the hardness of the welding nugget without the hardness recovery in the outside nugget's areas. There are very limited researches concerning the PWHT of FSWed 7075 aluminum alloy [16-20]. Barcelona et al. [16] have reported that PWHT of 7075 joint homogenized the
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distribution of precipitate particles in different areas. Nevertheless, results of a research conducted by Mahoney et al. [18] have shown that direct post-weld aging without solution treatment did not change the yield strength of 7075-T651 joint whereas degraded its fracture strength. Indeed, the most serious problem of the previous efforts is the occurrence of abnormal grain growth (AGG) phenomenon after heat treatment of welding zone which deteriorates mechanical properties such as tensile strength and hardness. It has been reported that the AGG in the welding area of 2195-T8 alloy has been taken place after solution treatment at 510 ˚C [21]. Similar results have been stated by Hassan et al. [22] in the case of 7010-T7 FSWed joints. They explained the source of AGG phenomenon as the dissolution of precipitate particles during FSW process. It has been shown that as the heat input during FSW increased, the probability of grain size enlargement after PWHT diminished [22]. We have recently developed a new method for heat treatment of precipitationhardenable Al alloys based on cyclic solution treatment (CST) followed by aging [23]. The positive effect of CST on mechanical properties of 6063 and 7075 Al alloys has been previously stated [24, 25]. For instance, approximately 22% enhancement in the hardness and strength of 7075-T6 alloy was obtained by utilizing CST compared with the conventional solution treatment [25]. In this paper, the effects of CST and aging on the hardness and tensile properties of FSWed 7075 joints are studied. Special attention is focused on the microstructure and grain growth of the FSWed joints after PWHT.
2. Materials and Methods A 5 mm thickness 7075-T6 Al alloy plate with chemical composition of Al-5.7 Zn-2.4 Mg-1.55 Cu-0.19 Cr-0.18 Fe (in wt.%) was used as the base material. A welding tool including a shoulder and a pin with diameters of 18 mm and 5 mm, respectively, was made from H13 steel. The FP4P milling machine with a fixed head and a moving table was used to
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weld the plates. The welding direction was perpendicular to the rolling direction of plates while the rotation direction was clockwise. The longitudinal and rotation speeds were 31.5 mm min-1 and 1250 rpm, respectively. Some of the samples were heat treated according to the procedure shown in Fig. 1. For cyclic solution treatment, the specimens were frequently heated between 400 ˚C and 480 ˚C for 1.5 h (0.25 h for each cycle). The heating and cooling rates were estimated to be ~ 20 ˚C s-1. After quenching in the water, the artificial aging was performed at 130 ˚C for various times up to 36 h. The microstructure of the samples was studied by using optical (OM) and electron microscopes (SEM and TEM). An Olympus BX60M optical microscope and a Vega/TescanXMU field emission scanning electron microscope (FESEM) equipped with an energydispersive spectroscopy (EDS) detector were used to examine the microstructure of the joints. Some of the specimens were etched using a solution consisted of 2.5 ml HNO3, 2 ml HF, 1.5 ml HCl and 95 ml water. The grain size of the samples was measured by using the image analysis software (Clemex, Version 3.5). The size and distribution of precipitates were assessed using a Philips EM 208 transmission electron microscope operated at 100 kV. TEM samples were cut from 2.5 mm depth of the welding surface and parallel to the welding direction by electro-discharge machining method. They were then mechanically polished to a thickness of ~ 50 µm by using a micro-cutter device. Finally, the thin samples were electrochemically polished in a twin-jet electropolisher (Struers, Tenupol-5) at 15 V by using a solution of 25% HNO3 in methanol and LN2. The Advance/Bruker AXS-D8 X-ray Diffractometer with Cu Kα radiation source (=1.15406 ºA) was used to identify the different phases. Microhardness measurements were performed using a Wolpert Vickers (V-Testor 2, D-6700) machine with applying a load of 50 gf for 15 s. Microhardness of the joints was measured in a plane perpendicular to the welding direction and a depth of 2.5 mm from their
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surface. The tensile test samples were prepared in a perpendicular direction to the welding direction according to ASTM-E8-04 standard (Fig. 2). The room temperature tensile tests were performed using the MTS-316 tensile machine with the strain rate of 1×10-3 s-1. Fracture surfaces of the FSWed samples after the tensile test were examined by using FESEM. Fig. 1. Fig. 2. 3. Results 3.1. Mechanical properties Fig. 3 illustrates the microhardness profiles of 7075 Al alloy joints perpendicular to the welding direction. The hardness in the welding zone of the FSWed part was slightly lower than that of the base metal (~ 158 HV) since the minimum of the hardness was located at the thermo-mechanical affected zone (TMAZ) and the HAZ. The hardness profile of the joint was roughly uniform after cyclic solution treatment with a hardness value of ~ 154±5 HV. A significant increase in the hardness level of the FSWed joint (~ 45%) after CST and aging for 24 h was noticed. Moreover, the average microhardness of the welding zone after precipitation hardening is approximately 26% higher than that of the base metal. Fig. 4 shows the microhardness variations of the CSTed joints versus aging time. It can be seen that the hardness of the welding zone increased by increasing the aging time and then started to decrease due to the overaging of precipitates. The peak hardness of 220±5 HV was achieved after 24 h aging which is approximately 42% higher than that of the CSTed condition. Fig. 3. Fig. 4. Tensile properties of 7075 Al alloy joints processed at different conditions are summarized in Table 1. The strength and ductility of the base plate were degraded after FSW processing. Interestingly, both strength and elongation were improved through CST; yield
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stress and ultimate tensile strength were improved ~ 7%, while the elongation to failure increased ~ 13%. A further enhancement in the strength of the FSWed joints was attained by aging treatment. It is clear from Table 1 that as the aging time increased to 24 h, the strength of the joint improved. After aging for 24 h, a high yield stress of 542 MPa and an ultimate tensile strength of 613 MPa were achieved that are ~ 33 % superior to those of the asprocessed specimen. Meanwhile, elongation of the joints after aging was almost comparable with that of the FSWed joint before heat treatment. The prolonged aging for 36 h resulted in a reduction in the strength values revealing the occurrence of overaging. Table 1. Fig. 5 shows the tensile fracture surfaces of the samples processed at various conditions. A large number of fine dimples are observed on the fracture surface of the FSWed sample (Fig. 5a) indicating the fine grain structure in the welding zone. This fracture appearance suggests the microviod coalescence including the formation, growth and coalescence of microvioids as the dominant fracture mechanism. The CST did not cause a considerable change in the fracture mechanism, though the size of dimples is smaller than that of the as-processed condition (Fig. 5b). The presence of many fine dimples in the fracture surface verifies the higher ductility of the CST specimen arisen from the solution of coarse precipitates. The fracture surface of the specimen aged for 24 h is rather similar to the CST specimen, except that the dimples are not well-developed (Fig. 5c). The inspection of the fracture surface of 36 h aged specimen demonstrated the less-developed dimples over the fracture surface (Fig. 5d). Some large precipitates were detected on the fracture surface, as shown by arrows in Fig. 5d. Fig. 5. 3.2. Microstructure
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Fig. 6 shows the OM microstructure of the FSWed 7075 Al alloy. The average grain sizes of the base metal and welding zone, determined by the line intercept method, are given in Table. 2. It should be noted that the grain size of the base metal was measured in the longitudinal direction. The coarse and elongated Al grains together with a dispersion of relatively coarse precipitates are observable in the OM micrograph of the base metal plate (Fig. 6 a-b). In contrast to the base metal, the microstructure of the welding zone consisted of very fine and equiaxed grains, as shown in Fig. 6 c-d. It is known that severe plastic deformation and the temperature rise of the welding zone upon FSW cause dynamic recrystallization phenomenon [26-28] that leads to a significant grain refinement of the Al alloy to ~ 4.4 µm. The uniform distribution of precipitates within the Al alloy matrix is seen as well. It is noteworthy that some rod-shaped precipitates been present in the base metal were diminished after FSW. This is primarily attributed to the severe plastic deformation imposed upon FSW processing which fractures the rod-shaped precipitates to spherical ones. Similar finding has been reported by Su et al. [4]. The structure and size of the grains in the welding zone did not significantly change after the post-weld CST (Fig. 6 e-f and Table. 2). This is an interesting finding since grain growth, which is likely to occur during the conventional solution treatment, was prohibited by the CST method. For instance, Hassan et al. [22] have observed a considerable grain growth of the welding zone from 1.7 to 150 µm by the conventional solution treatment at 485 ˚C for 1 h. As shown in Fig. 6 e-f, the precipitates were not completely dissolved in the Al matrix after CST. On the other hand, the grain structure of the CSTed sample was not altered after aging treatment, while a high fraction of second phase particles was dispersed within the Al alloy matrix, as shown in Fig. 6 g-h. Fig. 6. Table 2.
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The morphology and chemical composition of precipitates in the microstructure of 7075 Al alloy were studied by using FESEM and EDS analysis (Fig. 7). Fine precipitates including Al, Zn, and Mg elements were observed in the microstructure of the FSWed specimen. However, a low amount of fine particles enriched by Al, Cu, Mg, and Zn was detected in the microstructure of Al alloy after CST. FESEM images of the welding zone after CST and aging for 24 h show the fine dispersion of precipitate particles. EDS analysis confirms that precipitate particles include Al, Mg, Zn and Cu elements. Nevertheless, the formation of coarse precipitates with a relatively non-uniform distribution enriched by Al, Cu, and Fe elements was noticed for the 36 h aged sample. Complementary information on the size and distribution of the precipitates in the welding area after CST and aging for 24 h was obtained from TEM examination. As shown in Fig. 8, the nanoscale size (~ 5-100 nm) and spherical precipitate particles are located within the Al alloy grains and on the grain boundaries. Fig. 7. Fig. 8. . The XRD patterns of 7075 Al alloys processed at various conditions are represented in Fig. 9. All the XRD patterns show the typical Al reflections. In addition, the XRD patterns of the base plate and FSWed specimen show the presence of MgZn2 precipitates. On the other hand, MgAlCu phase was detected in the XRD pattern of the CSTed specimen. MgAlCu and Al7Cu2Mg precipitates were identified after aging of the CSTed specimens for 24 and 36 h, respectively. Fig. 9. 4. Discussion In contrast to the several previous investigations [20, 21, 29], the microstructural observations of FSWed 7075 Al alloy indicated that abnormal grain growth phenomenon has
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not taken place after the post-weld CST (Fig. 6 e-f). The occurrence of AGG has been attributed to the presence of grains with different sizes, grain boundaries with dissimilar migration rates, and strain gradients in the microstructure as well as the dissolution and growth of precipitate particles [21]. The experimental results pointed out that a uniform microstructure consisted of fine grains in the welding zone of 7075 Al alloy was obtained due to the incidence of dynamic recrystallization upon FSW that diminished the possibility of AGG. On the other hand, the presence of fine MgAlCu precipitate particles with an appropriate distribution significantly hindered the grain boundaries’ migration and prevented AGG. Sato et al. [30] have shown that AGG in 1100-H24 Al alloy happened when the maximum temperature of the heat treatment was higher than the temperature rise caused by FSW. While the maximum temperature of CST (480 ˚C) was less than the welding temperature (500 ˚C [21]), the probability of AGG phenomenon’s occurrence reduced. It should be noted that the welding area underwent solution treatment due to the temperature rise upon FSW that resulted in the partial or complete dissolution of precipitate particles. Furthermore, the cooling rate following FSW was not high enough to cause re-precipitation of the dissolved solute. Thus, only some small precipitates were formed (Fig. 6 c-d) that could not be effectively contributed to strengthening and resulted in the hardness reduction of the welded line. On the contrary, the temperature rise in HAZ and TMAZ was insufficient to cause the dissolution of precipitates, and lead to their coarsening [16] and a sharp drop in the hardness of the specimen. Similar results have been reported by Mishra et al. [29]. The hardness profile of the joint became more homogeneous with a considerable improvement in the hardness (154 HV) after CST, as shown in Fig. 3. This hardness value is comparable with the hardness of FSWed 7075 Al alloy after conventional solution treatment followed by artificial aging (T6) [17, 19], revealing the efficiency of CST (without aging) in the improvement of mechanical properties. The enhancement of tensile properties of 7075 Al
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alloy after CST compared with the base metal and FSWed plate was also detected (Table. 1). Accordingly, due to the cyclic nature of the heating above and below the solvus temperature during the solution treatment, it is believed that a simultaneous solutionizing and aging processes take place upon CST. In fact, as 7075 Al alloy solutionizes at the higher temperature (~ 480 ˚C), the large MgZn2 precipitates retained after FSW partially dissolve in the matrix. Subsequently, when the alloy transfers and heats at the lower temperature (~ 400 ˚C), the aging process occurs and fine metastable MgAlCu precipitates are formed. It should be noted that this process is taken place quickly since the solute atoms can diffuse easily at the high temperature. These two processes, i.e. the partial dissolution of large precipitates and the formation of fine precipitates take place consecutively in the next cycles of CST. Consequently, a uniform dispersion of nanoscale precipitates within 7075 Al alloy matrix is formed after CST which can efficiently contribute to strengthening. In addition, repetitive heating at two different temperatures during solution heat treatment induces thermal strains, thereby improving hardness and tensile strength. Following CST, metastable precipitates can transform to stable MgAlCu precipitates (Fig. 9) after artificial aging for 24 h at 130 ˚C. These precipitates can act as effective obstacles against the dislocations and enhance hardness and mechanical strength of the alloy. The peak-aged mechanical properties of FSWed 7075 Al alloy in this study are significantly superior to those of conventionally solution heat-treated and artificially aged [17, 19, 20, 26]. It is pertinent to point out that aging for 24 h not only improved the strength, but also did not reduce the elongation to failure of the alloy (see Table. 1). Mahoney et al. [18] reported that aging treatment degraded the ductility of the solution heat-treated FSWed 7075 Al alloy joints that was mainly caused by the coarsening of precipitate particles and the development of precipitation free zones (PFZs). The microstructural observations (Figs. 6-8) revealed that neither the coarsening of precipitates nor the formation of PFZs occurred after CST followed
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by 24 h aging, thereby the elongation of the alloy preserved and several dimples developed over the tensile fracture surface (Fig. 5c). In contrast, aging over 24 h resulted in a significant deterioration in both hardness and strength (Fig. 4 and Table. 1). Similar results have been reported by Kim et al [31]. According to Figs. 7j and 7k, the coarsening and agglomeration of precipitate particles occurred after aging for 36 h, revealing the overaging of the precipitates and formation of PFZs. On the other hand, the formation of brittle Al7Cu2Fe particles, can have a destructive effect on mechanical properties, especially ductility. Evaluation of the mechanical properties of precipitates in 7075 Al alloys by means of nanoindentation and micropillar compression tests have revealed that Al7Cu2Fe particles has the highest hardness and strength among the various inclusions [32, 33]. It was shown that Al7Cu2Fe are completely brittle particles with a high compressive failure strength of ~ 2.5 GPa which were simply fractured with no sign of plastic deformation [33]. Thus, elongation to failure of the overaged sample reduced, as it was confirmed by the SEM fracture surface (Fig. 5d).
5. Conclusions The effects of cyclic solution treatment (consecutive heating between 400 ˚C and 480 ˚C) followed by artificial aging on microstructure and mechanical properties of FSWed 7075 Al alloy were studied. It was found that FSW process deteriorated mechanical properties of the alloy, especially in HAZ and TMAZ. The CST extremely homogenized and recovered the mechanical properties of the joints without the occurrence of abnormal grain growth. The enhancement of mechanical properties via CST was attributed to the repetitive partial dissolution of large MgZn2 precipitates and formation of fine metastable MgAlCu precipitates. The preservation of the fine grain structure and the dispersion of fine stable MgAlCu precipitates after aging for 24 h at 130 ˚C caused a significant enhancement in
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hardness (~ 39 %), yield stress (~ 11 %), and ultimate tensile strength (~ 10 %) of the joints without a considerable change in the ductility compared with the base plate in T6 condition.
Reference [1]
M. Aissani, S. Gachi, F. Boubenider, Y. Benkedda, Design and optimization of friction stir welding tool, Materials and Manufacturing Processes 25 (11) (2010) 1199-1205.
[2]
P. Bahemmat, M. Haghpanahi, M. Besharati Givi, K. Reshad Seighalani, Study on dissimilar friction stir butt welding of AA7075-O and AA2024-t4 considering the manufacturing limitation, The International Journal of Advanced Manufacturing Technology 59 (9-12) (2012) 939-953.
[3]
R. Singh, C. Sharma, D. Dwivedi, N. Mehta, P. Kumar, The microstructure and mechanical properties of friction stir welded Al-Zn-Mg alloy in as welded and heat treated conditions, Materials and Design 32 (2) (2011) 682-687.
[4]
J.-Q. Su, T. Nelson, R. Mishra, M. Mahoney, Microstructural investigation of friction stir welded 7050 T651 aluminium, Acta Materialia 51 (3) (2003) 713-729.
[5]
R. Mishra, Z. Ma, Friction stir welding and processing, Materials Science and Engineering: R: Reports 50 (12) (2005) 1-78.
[6]
P. Cavaliere, E. Cerri, A. Squillace, Mechanical response of 2024-7075 aluminium alloys joined by friction stir welding, Journal of Materials Science 40 (14) (2005) 3669-3676.
[7]
K. Elangovan, V. Balasubramanian, Influences of post-weld heat treatment on tensile properties of friction stir-welded AA6061 aluminum alloy joints, Materials Characterization 59 (9) (2008) 1168-1177.
13
[8]
K. Krishnan, The effect of post weld heat treatment on the properties of 6061 friction stir welded joints, Journal of Materials Science 37 (3) (2002) 473-480.
[9]
W. D. Lockwood, B. Tomaz, A. Reynolds, Mechanical response of friction stir welded AA2024: experiment and modeling, Materials Science and Engineering: A 323 (12) (2002) 348-353.
[10]
S. Rajakumar, C. Muralidharan, V. Balasubramanian, Influence of friction stir welding process and tool parameters on strength properties of AA7075- T6 aluminium alloy joints, Materials and Design 32 (2) (2011) 535-549.
[11]
C. Rowe, W. Thomas, Advances in tooling materials for friction stir welding, TWI and Cedar Metals Ltd (2005) 1-11.
[12]
O. Hatamleh, P. M. Singh, H. Garmestani, Corrosion susceptibility of peened friction stir welded 7075 aluminum alloy joints, Corrosion Science 51 (1) (2009) 135-143.
[13]
J. C. Feng, Y. C. Chen, H. J. Liu, Effects of post-weld heat treatment on microstructure and mechanical properties of friction stir welded joints of 2219-O aluminium alloy, Materials Science and Technology 22 (1) (2006) 86-90.
[14]
R. Priya, V. Subramanya Sarma, K. Prasad Rao, Effect of post weld heat treatment on the microstructure and tensile properties of dissimilar friction stir welded AA2219 and AA6061 alloys, Transactions of the Indian Institute of Metals 62 (1) (2009) 1119.
[15]
Y. Sato, H. Kokawa, M. Enomoto, S. Jogan, T. Hashimoto, Precipitation sequence in friction stir weld of 6063 aluminum during aging, Metallurgical and Materials Transactions A 30 (12) (1999) 3125-3130.
[16]
A. Barcellona, G. Bua, L. Fratini, D. Palmeri, On microstructural phenomena occurring in friction stir welding of aluminium alloys, Journal of Materials Processing
14
Technology 177 (13) (2006) 340-343, proceedings of the 11th International Conference on Metal Forming 2006. [17]
G. pekolu, S. Erim, G. am, Effects of temper condition and post weld heat treatment on the microstructure and mechanical properties of friction stir butt-welded AA7075 Al alloy plates, The International Journal of Advanced Manufacturing Technology 70 (1-4) (2014) 201-213.
[18]
M. Mahoney, C. Rhodes, J. Flinto, W. Bingel, R. Spurling, Properties of friction-stirwelded 7075 T651 aluminum, Metallurgical and Materials Transactions A 29 (7) (1998) 1955-1964.
[19]
P. Sivaraj, D. Kanagarajan, V. Balasubramanian, Effect of post weld heat treatment on tensile properties and microstructure characteristics of friction stir welded armour grade AA7075-T651 aluminium alloy, Defence Technology 10 (1) (2014) 1-8.
[20]
Yeni, S. Sayer, O. Erturul, M. Pakdil, Effect of post-weld aging on the mechanical and microstructural properties of friction stir welded aluminum alloy 7075, Archives of Materials Science and Engineering 34 (2008) 105-109.
[21]
P. L. Threadgill, A. J. Leonard, H. R. Shercli, P. J. Withers, Friction stir welding of aluminium alloys, International Materials Reviews (2009) 49-93.
[22]
K. Hassan, A. Norman, D. Price, P. Prangnell, Stability of nugget zone grain structures in high strength Al-alloy friction stir welds during solution treatment, Acta Materialia 51 (7) (2003) 1923-1936.
[23]
H. Asgharzadeh and A. Simchi, A new procedure for precipitation hardening of nanostructured Al-Mg-Si alloys and their nanocomposites. Iranian patent, 57759, Publication Date: 2009.
15
[24]
H. Asgharzadeh, M. Bayazid, and A. Simchi, An investigation on the precipitation hardening of an ultrafine grained Al6063-Al2O3 nanocomposite. Powder Metallurgy World Congress and Exhibition (PM 2012), October 2012, Basel, Switzerland.
[25]
L. Radan, H.Ghorbani, O. dehkordi, S. M. Bayazid, H. Asgharzadeh. An investigation on the optimized precipitation hardening with Taguchi method and microstructure of 7075 aluminum alloy.
3th Ultrafine Grained and Nanostructured
Materials
(UFGNSM2013), November 2013, Tehran, Iran. [26]
C. B. Fuller, M. W. Mahoney, M. Calabrese, L. Micona, Evolution of microstructure and mechanical properties in naturally aged 7050 and 7075 Al friction stir welds, Materials Science and Engineering: A 527 (9) (2010) 2233-2240.
[27]
F. Gemme, Y. Verreman, L. Dubourg, P. Wanjara, Effect of welding parameters on microstructure and mechanical properties of AA7075-T6 friction stir welded joints, Fatigue and Fracture of Engineering Materials and Structures 34 (11) (2011) 877-886.
[28]
S. Ren, Z. Ma, L. Chen, Effect of welding parameters on tensile properties and fracture behavior of friction stir welded Al-Mg-Si alloy, Scripta Materialia 56 (1) (2007) 69-72.
[29]
R. Mishra, M. Mahoney, Friction Stir Welding and Processing, ASM International, 2007.
[30]
Y. Sato, S. Park, A. Matsunaga, A. Honda, H. Kokawa, Novel production for highly formable Mg alloy plate, Journal of Materials Science 40 (3) (2005) 637-642.
[31]
H. H. Kim, C. G. Kang, Evaluation of precipitation hardening characteristics of rheology-forged Al 7075 aluminum alloy using nano-or microindentation and atomic force microscopy, Metallurgical and Materials Transactions A 41 (3) (2010) 696-705.
[32]
S. S. Singh, C. Schwartzstein, J. J. Williams, X. Xiao, F. De Carlo, N. Chawla, 3D microstructural characterization and mechanical properties of constituent particles in
16
Al 7075 alloys using X-ray synchrotron tomography and nanoindentation, Journal of Alloys and Compounds 602 (2014) 163-174. [33]
S. S. Singh, E. Guo, H. Xie, N. Chawla, Mechanical properties of intermetallic inclusions in Al 7075 alloys by micropillar compression, Intermetallics 62 (2015) 6975.
Figure captions Fig. 1. Schematic illustration of the stages of PWHT including CST and aging. Fig. 2. Scheme of the welding and the tensile sample. Fig. 3. Microhardness profiles in the weld joints transverse to the welding direction. Fig. 4. The microhardness changes of the stir zone as a function of the aging time. The CSTed 7075 alloy joints were aged at 130 ˚C. Fig. 5. SEM fracture surfaces of FSWed 7075 Al alloy joints after tensile testing: (a) asprocessed, (b) CSTed, (c) CSTed and aged for 24 h and (d) CSTed and aged for 36 h. Fig. 6. Optical microstructure of FSWed 7075 Al alloy: (a, b) base metal, (c, d) welding zone, (e, f) welding zone after CST and (g, h) welding zone after CST and aging for 24 h. Fig. 7. FESEM images and EDS analyses from the welding zone of FSWed 7075 Al alloy showing the distribution of precipitates: (a-c) as-processed, (d-f) CSTed, (g-i) CSTed and aged for 24 h, and (j-l) CSTed and aged for 36 h. Fig. 8. TEM image of the microstructure of the FSWed joint after CST and aging for 24 h. Fig. 9. XRD patterns of 7075 Al alloy: (a) base metal, (b) as-processed, (c) CSTed, (d) CSTed and aged for 24 h, (e) CSTed and aged for 36 h. Table captions Table 1. Tensile properties of the base metal and FSWed 7075 Al alloy joints before and after age hardening. Table 2. Grain size of Al in the FSWed 7075 Al alloy before and after heat treatment.
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
CSTed+Aged Specimen
Tensile yield stress (MPa)
Base plate
489±15
FSWed
402±10
Aging time (h) 0
6
12
18
24
36
431±11
463±11
476±12
491±16
542±12
436±6
47
Ultimate tensile strength (MPa)
559±10
460±13
490±10
525±11
544±9
558±9
613±12
503±11
Elongation (%)
11.5±1
10.3±2
11.7±1
10.0±1
10.4±2
10.4±0.5
10.0±0.5
6.3±0.5
Equivalent diameter (μm)
Specimen Base Metal
140.1
Welding zone
4.4
Welding zone after CST
5.3
Welding zone after CST and aging for 24 h
6.54
48
PII: DOI: Reference:
S0921-5093(15)30473-1 http://dx.doi.org/10.1016/j.msea.2015.10.010 MSA32861
To appear in: Materials Science & Engineering A Received date: 10 July 2015 Revised date: 2 October 2015 Accepted date: 3 October 2015 Cite this article as: S.M. Bayazid, H. Farhangi, H. Asgharzadeh, L. Radan, A. Ghahramani and A. Mirhaji, Effect of cyclic solution treatment on microstructure and mechanical properties of friction stir welded 7075 Al alloy, Materials Science & Engineering A, http://dx.doi.org/10.1016/j.msea.2015.10.010 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Effect of cyclic solution treatment on microstructure and mechanical properties of friction stir welded 7075 Al alloy
S.M. Bayazid1*, H. Farhangi2, H. Asgharzadeh3, L. Radan4, A. Ghahramani5, A. Mirhaji6
1,2,5
School of Metallurgy and Materials, College of Engineering, University of Tehran,
Tehran, P.O. 11155-4563, Iran 3,6
Department of Materials Engineering, University of Tabriz, P.O. Box 51666-16471,
Tabriz, Iran 4
Department of Materials Science and Engineering, School of Engineering, Shiraz
University, P.O. Box 71348-51154, Shiraz, Iran
* Corresponding author. Email: [email protected] Cell phone number: +98 9141485543
1
Abstract 7075-T6 aluminum alloy plates were prepared by friction stir welding (FSW) followed by age hardening. A novel solutionizing method, namely cyclic solution treatment (CST), comprising of a repeated heating between 400 and 480 ˚C for 0.25 h was employed. The microstructure of the joints was studied by optical microscopy (OM), scanning electron microscopy (SEM) and transmission electron microscopy (TEM) techniques. The effect of CST on mechanical properties was assessed by means of tensile test and microhardness measurement. A significant grain size refinement is taken place by FSW whilst the grain size is not considerably changed after CST. The results show that precipitate particles of the welding area before and after heat treatment are MgZn2 and MgAlCu/Al7Cu2Fe, respectively. CST improves tensile strength and elongation while homogenizes the hardness distribution of the FSWed joint. A noteworthy enhancement in the hardness (~ 45%) and tensile strength (~ 33%) of the FSWed sample is achieved after CST and aging at 130 ˚C for 24 h. The tensile fracture surface of the Al alloy joint demonstrates fine dimples after CST while lessdeveloped dimples are detected after aging.
Keywords: 7075 Al alloy; Friction stir welding; Cyclic solution treatment; Aging; Microstructure; Mechanical properties
2
1. Introduction 7xxx series aluminum alloys have a wide variety of applications in different industries such as aerospace and automotive due to their high strength-to-weight ratio. However, these alloys are difficult to join with fusion welding methods which produce solidification cracks and high volume of porosities [1-4]. Accordingly, friction stir welding (FSW) as a solid state joining method is being noticed recently by many industries and researchers due to its capability to reduce defects compared with conventional techniques [5]. Although there is a large body of research in the literature on FSW of 7075 Al alloy [3, 5, 6], however, many problems still remain unsolved related to poor mechanical properties of weldment’s heat affected zone (HAZ) [7-9]. Recently, some efforts have been devoted to improving the mechanical properties of the FSWed joints like changing the welding and tooling parameters (e.g. rotational and longitudinal speed, size of the pin and geometry of the tool) [10, 11] and applying shot-peening or laser-peening prior to welding [12]. Among the various methods, post-weld heat treatment (PWHT) is a versatile and inexpensive technique for the recovery of degraded mechanical properties of FSWed parts [13-15]. Sato and Kakawa [15] have shown that the yield and tensile strength of FSWed 6063-T5 Al alloy increased after solution treatment at 530 ˚C for 1 h followed by water quenching and aging at 175 ˚C for 12 h. Feng et al. [13] have reported that PWHT of FSWed 2219-O alloy has a considerable effect on the fracture locations of the joints. Meanwhile, the tensile strength of the weldment can be improved by increasing the solutionizing temperature. Priya et al. [14] have stated that aging of FSWed 2219 and 6061-T6 alloys at 165 ˚C for 18 h without solution treatment results in a considerable improvement in the hardness of the welding nugget without the hardness recovery in the outside nugget's areas. There are very limited researches concerning the PWHT of FSWed 7075 aluminum alloy [16-20]. Barcelona et al. [16] have reported that PWHT of 7075 joint homogenized the
3
distribution of precipitate particles in different areas. Nevertheless, results of a research conducted by Mahoney et al. [18] have shown that direct post-weld aging without solution treatment did not change the yield strength of 7075-T651 joint whereas degraded its fracture strength. Indeed, the most serious problem of the previous efforts is the occurrence of abnormal grain growth (AGG) phenomenon after heat treatment of welding zone which deteriorates mechanical properties such as tensile strength and hardness. It has been reported that the AGG in the welding area of 2195-T8 alloy has been taken place after solution treatment at 510 ˚C [21]. Similar results have been stated by Hassan et al. [22] in the case of 7010-T7 FSWed joints. They explained the source of AGG phenomenon as the dissolution of precipitate particles during FSW process. It has been shown that as the heat input during FSW increased, the probability of grain size enlargement after PWHT diminished [22]. We have recently developed a new method for heat treatment of precipitationhardenable Al alloys based on cyclic solution treatment (CST) followed by aging [23]. The positive effect of CST on mechanical properties of 6063 and 7075 Al alloys has been previously stated [24, 25]. For instance, approximately 22% enhancement in the hardness and strength of 7075-T6 alloy was obtained by utilizing CST compared with the conventional solution treatment [25]. In this paper, the effects of CST and aging on the hardness and tensile properties of FSWed 7075 joints are studied. Special attention is focused on the microstructure and grain growth of the FSWed joints after PWHT.
2. Materials and Methods A 5 mm thickness 7075-T6 Al alloy plate with chemical composition of Al-5.7 Zn-2.4 Mg-1.55 Cu-0.19 Cr-0.18 Fe (in wt.%) was used as the base material. A welding tool including a shoulder and a pin with diameters of 18 mm and 5 mm, respectively, was made from H13 steel. The FP4P milling machine with a fixed head and a moving table was used to
4
weld the plates. The welding direction was perpendicular to the rolling direction of plates while the rotation direction was clockwise. The longitudinal and rotation speeds were 31.5 mm min-1 and 1250 rpm, respectively. Some of the samples were heat treated according to the procedure shown in Fig. 1. For cyclic solution treatment, the specimens were frequently heated between 400 ˚C and 480 ˚C for 1.5 h (0.25 h for each cycle). The heating and cooling rates were estimated to be ~ 20 ˚C s-1. After quenching in the water, the artificial aging was performed at 130 ˚C for various times up to 36 h. The microstructure of the samples was studied by using optical (OM) and electron microscopes (SEM and TEM). An Olympus BX60M optical microscope and a Vega/TescanXMU field emission scanning electron microscope (FESEM) equipped with an energydispersive spectroscopy (EDS) detector were used to examine the microstructure of the joints. Some of the specimens were etched using a solution consisted of 2.5 ml HNO3, 2 ml HF, 1.5 ml HCl and 95 ml water. The grain size of the samples was measured by using the image analysis software (Clemex, Version 3.5). The size and distribution of precipitates were assessed using a Philips EM 208 transmission electron microscope operated at 100 kV. TEM samples were cut from 2.5 mm depth of the welding surface and parallel to the welding direction by electro-discharge machining method. They were then mechanically polished to a thickness of ~ 50 µm by using a micro-cutter device. Finally, the thin samples were electrochemically polished in a twin-jet electropolisher (Struers, Tenupol-5) at 15 V by using a solution of 25% HNO3 in methanol and LN2. The Advance/Bruker AXS-D8 X-ray Diffractometer with Cu Kα radiation source (=1.15406 ºA) was used to identify the different phases. Microhardness measurements were performed using a Wolpert Vickers (V-Testor 2, D-6700) machine with applying a load of 50 gf for 15 s. Microhardness of the joints was measured in a plane perpendicular to the welding direction and a depth of 2.5 mm from their
5
surface. The tensile test samples were prepared in a perpendicular direction to the welding direction according to ASTM-E8-04 standard (Fig. 2). The room temperature tensile tests were performed using the MTS-316 tensile machine with the strain rate of 1×10-3 s-1. Fracture surfaces of the FSWed samples after the tensile test were examined by using FESEM. Fig. 1. Fig. 2. 3. Results 3.1. Mechanical properties Fig. 3 illustrates the microhardness profiles of 7075 Al alloy joints perpendicular to the welding direction. The hardness in the welding zone of the FSWed part was slightly lower than that of the base metal (~ 158 HV) since the minimum of the hardness was located at the thermo-mechanical affected zone (TMAZ) and the HAZ. The hardness profile of the joint was roughly uniform after cyclic solution treatment with a hardness value of ~ 154±5 HV. A significant increase in the hardness level of the FSWed joint (~ 45%) after CST and aging for 24 h was noticed. Moreover, the average microhardness of the welding zone after precipitation hardening is approximately 26% higher than that of the base metal. Fig. 4 shows the microhardness variations of the CSTed joints versus aging time. It can be seen that the hardness of the welding zone increased by increasing the aging time and then started to decrease due to the overaging of precipitates. The peak hardness of 220±5 HV was achieved after 24 h aging which is approximately 42% higher than that of the CSTed condition. Fig. 3. Fig. 4. Tensile properties of 7075 Al alloy joints processed at different conditions are summarized in Table 1. The strength and ductility of the base plate were degraded after FSW processing. Interestingly, both strength and elongation were improved through CST; yield
6
stress and ultimate tensile strength were improved ~ 7%, while the elongation to failure increased ~ 13%. A further enhancement in the strength of the FSWed joints was attained by aging treatment. It is clear from Table 1 that as the aging time increased to 24 h, the strength of the joint improved. After aging for 24 h, a high yield stress of 542 MPa and an ultimate tensile strength of 613 MPa were achieved that are ~ 33 % superior to those of the asprocessed specimen. Meanwhile, elongation of the joints after aging was almost comparable with that of the FSWed joint before heat treatment. The prolonged aging for 36 h resulted in a reduction in the strength values revealing the occurrence of overaging. Table 1. Fig. 5 shows the tensile fracture surfaces of the samples processed at various conditions. A large number of fine dimples are observed on the fracture surface of the FSWed sample (Fig. 5a) indicating the fine grain structure in the welding zone. This fracture appearance suggests the microviod coalescence including the formation, growth and coalescence of microvioids as the dominant fracture mechanism. The CST did not cause a considerable change in the fracture mechanism, though the size of dimples is smaller than that of the as-processed condition (Fig. 5b). The presence of many fine dimples in the fracture surface verifies the higher ductility of the CST specimen arisen from the solution of coarse precipitates. The fracture surface of the specimen aged for 24 h is rather similar to the CST specimen, except that the dimples are not well-developed (Fig. 5c). The inspection of the fracture surface of 36 h aged specimen demonstrated the less-developed dimples over the fracture surface (Fig. 5d). Some large precipitates were detected on the fracture surface, as shown by arrows in Fig. 5d. Fig. 5. 3.2. Microstructure
7
Fig. 6 shows the OM microstructure of the FSWed 7075 Al alloy. The average grain sizes of the base metal and welding zone, determined by the line intercept method, are given in Table. 2. It should be noted that the grain size of the base metal was measured in the longitudinal direction. The coarse and elongated Al grains together with a dispersion of relatively coarse precipitates are observable in the OM micrograph of the base metal plate (Fig. 6 a-b). In contrast to the base metal, the microstructure of the welding zone consisted of very fine and equiaxed grains, as shown in Fig. 6 c-d. It is known that severe plastic deformation and the temperature rise of the welding zone upon FSW cause dynamic recrystallization phenomenon [26-28] that leads to a significant grain refinement of the Al alloy to ~ 4.4 µm. The uniform distribution of precipitates within the Al alloy matrix is seen as well. It is noteworthy that some rod-shaped precipitates been present in the base metal were diminished after FSW. This is primarily attributed to the severe plastic deformation imposed upon FSW processing which fractures the rod-shaped precipitates to spherical ones. Similar finding has been reported by Su et al. [4]. The structure and size of the grains in the welding zone did not significantly change after the post-weld CST (Fig. 6 e-f and Table. 2). This is an interesting finding since grain growth, which is likely to occur during the conventional solution treatment, was prohibited by the CST method. For instance, Hassan et al. [22] have observed a considerable grain growth of the welding zone from 1.7 to 150 µm by the conventional solution treatment at 485 ˚C for 1 h. As shown in Fig. 6 e-f, the precipitates were not completely dissolved in the Al matrix after CST. On the other hand, the grain structure of the CSTed sample was not altered after aging treatment, while a high fraction of second phase particles was dispersed within the Al alloy matrix, as shown in Fig. 6 g-h. Fig. 6. Table 2.
8
The morphology and chemical composition of precipitates in the microstructure of 7075 Al alloy were studied by using FESEM and EDS analysis (Fig. 7). Fine precipitates including Al, Zn, and Mg elements were observed in the microstructure of the FSWed specimen. However, a low amount of fine particles enriched by Al, Cu, Mg, and Zn was detected in the microstructure of Al alloy after CST. FESEM images of the welding zone after CST and aging for 24 h show the fine dispersion of precipitate particles. EDS analysis confirms that precipitate particles include Al, Mg, Zn and Cu elements. Nevertheless, the formation of coarse precipitates with a relatively non-uniform distribution enriched by Al, Cu, and Fe elements was noticed for the 36 h aged sample. Complementary information on the size and distribution of the precipitates in the welding area after CST and aging for 24 h was obtained from TEM examination. As shown in Fig. 8, the nanoscale size (~ 5-100 nm) and spherical precipitate particles are located within the Al alloy grains and on the grain boundaries. Fig. 7. Fig. 8. . The XRD patterns of 7075 Al alloys processed at various conditions are represented in Fig. 9. All the XRD patterns show the typical Al reflections. In addition, the XRD patterns of the base plate and FSWed specimen show the presence of MgZn2 precipitates. On the other hand, MgAlCu phase was detected in the XRD pattern of the CSTed specimen. MgAlCu and Al7Cu2Mg precipitates were identified after aging of the CSTed specimens for 24 and 36 h, respectively. Fig. 9. 4. Discussion In contrast to the several previous investigations [20, 21, 29], the microstructural observations of FSWed 7075 Al alloy indicated that abnormal grain growth phenomenon has
9
not taken place after the post-weld CST (Fig. 6 e-f). The occurrence of AGG has been attributed to the presence of grains with different sizes, grain boundaries with dissimilar migration rates, and strain gradients in the microstructure as well as the dissolution and growth of precipitate particles [21]. The experimental results pointed out that a uniform microstructure consisted of fine grains in the welding zone of 7075 Al alloy was obtained due to the incidence of dynamic recrystallization upon FSW that diminished the possibility of AGG. On the other hand, the presence of fine MgAlCu precipitate particles with an appropriate distribution significantly hindered the grain boundaries’ migration and prevented AGG. Sato et al. [30] have shown that AGG in 1100-H24 Al alloy happened when the maximum temperature of the heat treatment was higher than the temperature rise caused by FSW. While the maximum temperature of CST (480 ˚C) was less than the welding temperature (500 ˚C [21]), the probability of AGG phenomenon’s occurrence reduced. It should be noted that the welding area underwent solution treatment due to the temperature rise upon FSW that resulted in the partial or complete dissolution of precipitate particles. Furthermore, the cooling rate following FSW was not high enough to cause re-precipitation of the dissolved solute. Thus, only some small precipitates were formed (Fig. 6 c-d) that could not be effectively contributed to strengthening and resulted in the hardness reduction of the welded line. On the contrary, the temperature rise in HAZ and TMAZ was insufficient to cause the dissolution of precipitates, and lead to their coarsening [16] and a sharp drop in the hardness of the specimen. Similar results have been reported by Mishra et al. [29]. The hardness profile of the joint became more homogeneous with a considerable improvement in the hardness (154 HV) after CST, as shown in Fig. 3. This hardness value is comparable with the hardness of FSWed 7075 Al alloy after conventional solution treatment followed by artificial aging (T6) [17, 19], revealing the efficiency of CST (without aging) in the improvement of mechanical properties. The enhancement of tensile properties of 7075 Al
10
alloy after CST compared with the base metal and FSWed plate was also detected (Table. 1). Accordingly, due to the cyclic nature of the heating above and below the solvus temperature during the solution treatment, it is believed that a simultaneous solutionizing and aging processes take place upon CST. In fact, as 7075 Al alloy solutionizes at the higher temperature (~ 480 ˚C), the large MgZn2 precipitates retained after FSW partially dissolve in the matrix. Subsequently, when the alloy transfers and heats at the lower temperature (~ 400 ˚C), the aging process occurs and fine metastable MgAlCu precipitates are formed. It should be noted that this process is taken place quickly since the solute atoms can diffuse easily at the high temperature. These two processes, i.e. the partial dissolution of large precipitates and the formation of fine precipitates take place consecutively in the next cycles of CST. Consequently, a uniform dispersion of nanoscale precipitates within 7075 Al alloy matrix is formed after CST which can efficiently contribute to strengthening. In addition, repetitive heating at two different temperatures during solution heat treatment induces thermal strains, thereby improving hardness and tensile strength. Following CST, metastable precipitates can transform to stable MgAlCu precipitates (Fig. 9) after artificial aging for 24 h at 130 ˚C. These precipitates can act as effective obstacles against the dislocations and enhance hardness and mechanical strength of the alloy. The peak-aged mechanical properties of FSWed 7075 Al alloy in this study are significantly superior to those of conventionally solution heat-treated and artificially aged [17, 19, 20, 26]. It is pertinent to point out that aging for 24 h not only improved the strength, but also did not reduce the elongation to failure of the alloy (see Table. 1). Mahoney et al. [18] reported that aging treatment degraded the ductility of the solution heat-treated FSWed 7075 Al alloy joints that was mainly caused by the coarsening of precipitate particles and the development of precipitation free zones (PFZs). The microstructural observations (Figs. 6-8) revealed that neither the coarsening of precipitates nor the formation of PFZs occurred after CST followed
11
by 24 h aging, thereby the elongation of the alloy preserved and several dimples developed over the tensile fracture surface (Fig. 5c). In contrast, aging over 24 h resulted in a significant deterioration in both hardness and strength (Fig. 4 and Table. 1). Similar results have been reported by Kim et al [31]. According to Figs. 7j and 7k, the coarsening and agglomeration of precipitate particles occurred after aging for 36 h, revealing the overaging of the precipitates and formation of PFZs. On the other hand, the formation of brittle Al7Cu2Fe particles, can have a destructive effect on mechanical properties, especially ductility. Evaluation of the mechanical properties of precipitates in 7075 Al alloys by means of nanoindentation and micropillar compression tests have revealed that Al7Cu2Fe particles has the highest hardness and strength among the various inclusions [32, 33]. It was shown that Al7Cu2Fe are completely brittle particles with a high compressive failure strength of ~ 2.5 GPa which were simply fractured with no sign of plastic deformation [33]. Thus, elongation to failure of the overaged sample reduced, as it was confirmed by the SEM fracture surface (Fig. 5d).
5. Conclusions The effects of cyclic solution treatment (consecutive heating between 400 ˚C and 480 ˚C) followed by artificial aging on microstructure and mechanical properties of FSWed 7075 Al alloy were studied. It was found that FSW process deteriorated mechanical properties of the alloy, especially in HAZ and TMAZ. The CST extremely homogenized and recovered the mechanical properties of the joints without the occurrence of abnormal grain growth. The enhancement of mechanical properties via CST was attributed to the repetitive partial dissolution of large MgZn2 precipitates and formation of fine metastable MgAlCu precipitates. The preservation of the fine grain structure and the dispersion of fine stable MgAlCu precipitates after aging for 24 h at 130 ˚C caused a significant enhancement in
12
hardness (~ 39 %), yield stress (~ 11 %), and ultimate tensile strength (~ 10 %) of the joints without a considerable change in the ductility compared with the base plate in T6 condition.
Reference [1]
M. Aissani, S. Gachi, F. Boubenider, Y. Benkedda, Design and optimization of friction stir welding tool, Materials and Manufacturing Processes 25 (11) (2010) 1199-1205.
[2]
P. Bahemmat, M. Haghpanahi, M. Besharati Givi, K. Reshad Seighalani, Study on dissimilar friction stir butt welding of AA7075-O and AA2024-t4 considering the manufacturing limitation, The International Journal of Advanced Manufacturing Technology 59 (9-12) (2012) 939-953.
[3]
R. Singh, C. Sharma, D. Dwivedi, N. Mehta, P. Kumar, The microstructure and mechanical properties of friction stir welded Al-Zn-Mg alloy in as welded and heat treated conditions, Materials and Design 32 (2) (2011) 682-687.
[4]
J.-Q. Su, T. Nelson, R. Mishra, M. Mahoney, Microstructural investigation of friction stir welded 7050 T651 aluminium, Acta Materialia 51 (3) (2003) 713-729.
[5]
R. Mishra, Z. Ma, Friction stir welding and processing, Materials Science and Engineering: R: Reports 50 (12) (2005) 1-78.
[6]
P. Cavaliere, E. Cerri, A. Squillace, Mechanical response of 2024-7075 aluminium alloys joined by friction stir welding, Journal of Materials Science 40 (14) (2005) 3669-3676.
[7]
K. Elangovan, V. Balasubramanian, Influences of post-weld heat treatment on tensile properties of friction stir-welded AA6061 aluminum alloy joints, Materials Characterization 59 (9) (2008) 1168-1177.
13
[8]
K. Krishnan, The effect of post weld heat treatment on the properties of 6061 friction stir welded joints, Journal of Materials Science 37 (3) (2002) 473-480.
[9]
W. D. Lockwood, B. Tomaz, A. Reynolds, Mechanical response of friction stir welded AA2024: experiment and modeling, Materials Science and Engineering: A 323 (12) (2002) 348-353.
[10]
S. Rajakumar, C. Muralidharan, V. Balasubramanian, Influence of friction stir welding process and tool parameters on strength properties of AA7075- T6 aluminium alloy joints, Materials and Design 32 (2) (2011) 535-549.
[11]
C. Rowe, W. Thomas, Advances in tooling materials for friction stir welding, TWI and Cedar Metals Ltd (2005) 1-11.
[12]
O. Hatamleh, P. M. Singh, H. Garmestani, Corrosion susceptibility of peened friction stir welded 7075 aluminum alloy joints, Corrosion Science 51 (1) (2009) 135-143.
[13]
J. C. Feng, Y. C. Chen, H. J. Liu, Effects of post-weld heat treatment on microstructure and mechanical properties of friction stir welded joints of 2219-O aluminium alloy, Materials Science and Technology 22 (1) (2006) 86-90.
[14]
R. Priya, V. Subramanya Sarma, K. Prasad Rao, Effect of post weld heat treatment on the microstructure and tensile properties of dissimilar friction stir welded AA2219 and AA6061 alloys, Transactions of the Indian Institute of Metals 62 (1) (2009) 1119.
[15]
Y. Sato, H. Kokawa, M. Enomoto, S. Jogan, T. Hashimoto, Precipitation sequence in friction stir weld of 6063 aluminum during aging, Metallurgical and Materials Transactions A 30 (12) (1999) 3125-3130.
[16]
A. Barcellona, G. Bua, L. Fratini, D. Palmeri, On microstructural phenomena occurring in friction stir welding of aluminium alloys, Journal of Materials Processing
14
Technology 177 (13) (2006) 340-343, proceedings of the 11th International Conference on Metal Forming 2006. [17]
G. pekolu, S. Erim, G. am, Effects of temper condition and post weld heat treatment on the microstructure and mechanical properties of friction stir butt-welded AA7075 Al alloy plates, The International Journal of Advanced Manufacturing Technology 70 (1-4) (2014) 201-213.
[18]
M. Mahoney, C. Rhodes, J. Flinto, W. Bingel, R. Spurling, Properties of friction-stirwelded 7075 T651 aluminum, Metallurgical and Materials Transactions A 29 (7) (1998) 1955-1964.
[19]
P. Sivaraj, D. Kanagarajan, V. Balasubramanian, Effect of post weld heat treatment on tensile properties and microstructure characteristics of friction stir welded armour grade AA7075-T651 aluminium alloy, Defence Technology 10 (1) (2014) 1-8.
[20]
Yeni, S. Sayer, O. Erturul, M. Pakdil, Effect of post-weld aging on the mechanical and microstructural properties of friction stir welded aluminum alloy 7075, Archives of Materials Science and Engineering 34 (2008) 105-109.
[21]
P. L. Threadgill, A. J. Leonard, H. R. Shercli, P. J. Withers, Friction stir welding of aluminium alloys, International Materials Reviews (2009) 49-93.
[22]
K. Hassan, A. Norman, D. Price, P. Prangnell, Stability of nugget zone grain structures in high strength Al-alloy friction stir welds during solution treatment, Acta Materialia 51 (7) (2003) 1923-1936.
[23]
H. Asgharzadeh and A. Simchi, A new procedure for precipitation hardening of nanostructured Al-Mg-Si alloys and their nanocomposites. Iranian patent, 57759, Publication Date: 2009.
15
[24]
H. Asgharzadeh, M. Bayazid, and A. Simchi, An investigation on the precipitation hardening of an ultrafine grained Al6063-Al2O3 nanocomposite. Powder Metallurgy World Congress and Exhibition (PM 2012), October 2012, Basel, Switzerland.
[25]
L. Radan, H.Ghorbani, O. dehkordi, S. M. Bayazid, H. Asgharzadeh. An investigation on the optimized precipitation hardening with Taguchi method and microstructure of 7075 aluminum alloy.
3th Ultrafine Grained and Nanostructured
Materials
(UFGNSM2013), November 2013, Tehran, Iran. [26]
C. B. Fuller, M. W. Mahoney, M. Calabrese, L. Micona, Evolution of microstructure and mechanical properties in naturally aged 7050 and 7075 Al friction stir welds, Materials Science and Engineering: A 527 (9) (2010) 2233-2240.
[27]
F. Gemme, Y. Verreman, L. Dubourg, P. Wanjara, Effect of welding parameters on microstructure and mechanical properties of AA7075-T6 friction stir welded joints, Fatigue and Fracture of Engineering Materials and Structures 34 (11) (2011) 877-886.
[28]
S. Ren, Z. Ma, L. Chen, Effect of welding parameters on tensile properties and fracture behavior of friction stir welded Al-Mg-Si alloy, Scripta Materialia 56 (1) (2007) 69-72.
[29]
R. Mishra, M. Mahoney, Friction Stir Welding and Processing, ASM International, 2007.
[30]
Y. Sato, S. Park, A. Matsunaga, A. Honda, H. Kokawa, Novel production for highly formable Mg alloy plate, Journal of Materials Science 40 (3) (2005) 637-642.
[31]
H. H. Kim, C. G. Kang, Evaluation of precipitation hardening characteristics of rheology-forged Al 7075 aluminum alloy using nano-or microindentation and atomic force microscopy, Metallurgical and Materials Transactions A 41 (3) (2010) 696-705.
[32]
S. S. Singh, C. Schwartzstein, J. J. Williams, X. Xiao, F. De Carlo, N. Chawla, 3D microstructural characterization and mechanical properties of constituent particles in
16
Al 7075 alloys using X-ray synchrotron tomography and nanoindentation, Journal of Alloys and Compounds 602 (2014) 163-174. [33]
S. S. Singh, E. Guo, H. Xie, N. Chawla, Mechanical properties of intermetallic inclusions in Al 7075 alloys by micropillar compression, Intermetallics 62 (2015) 6975.
Figure captions Fig. 1. Schematic illustration of the stages of PWHT including CST and aging. Fig. 2. Scheme of the welding and the tensile sample. Fig. 3. Microhardness profiles in the weld joints transverse to the welding direction. Fig. 4. The microhardness changes of the stir zone as a function of the aging time. The CSTed 7075 alloy joints were aged at 130 ˚C. Fig. 5. SEM fracture surfaces of FSWed 7075 Al alloy joints after tensile testing: (a) asprocessed, (b) CSTed, (c) CSTed and aged for 24 h and (d) CSTed and aged for 36 h. Fig. 6. Optical microstructure of FSWed 7075 Al alloy: (a, b) base metal, (c, d) welding zone, (e, f) welding zone after CST and (g, h) welding zone after CST and aging for 24 h. Fig. 7. FESEM images and EDS analyses from the welding zone of FSWed 7075 Al alloy showing the distribution of precipitates: (a-c) as-processed, (d-f) CSTed, (g-i) CSTed and aged for 24 h, and (j-l) CSTed and aged for 36 h. Fig. 8. TEM image of the microstructure of the FSWed joint after CST and aging for 24 h. Fig. 9. XRD patterns of 7075 Al alloy: (a) base metal, (b) as-processed, (c) CSTed, (d) CSTed and aged for 24 h, (e) CSTed and aged for 36 h. Table captions Table 1. Tensile properties of the base metal and FSWed 7075 Al alloy joints before and after age hardening. Table 2. Grain size of Al in the FSWed 7075 Al alloy before and after heat treatment.
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
CSTed+Aged Specimen
Tensile yield stress (MPa)
Base plate
489±15
FSWed
402±10
Aging time (h) 0
6
12
18
24
36
431±11
463±11
476±12
491±16
542±12
436±6
47
Ultimate tensile strength (MPa)
559±10
460±13
490±10
525±11
544±9
558±9
613±12
503±11
Elongation (%)
11.5±1
10.3±2
11.7±1
10.0±1
10.4±2
10.4±0.5
10.0±0.5
6.3±0.5
Equivalent diameter (μm)
Specimen Base Metal
140.1
Welding zone
4.4
Welding zone after CST
5.3
Welding zone after CST and aging for 24 h
6.54
48